September 15, 2005

Y chromosome perspective on Mediterranean populations

Capelli et al. have written an important new article on Y-chromosomal variation in the Mediterranean basin. This is the most comprehensive study yet on the region, using a combination of biallelic polymorphisms defining haplogroups and microsatellites over several Mediterranean populations, including many population samples taken from the literature. Moreover, mtDNA and autosomal data are also included, and these tend to support the authors' broad findings.

The key finding is that Mediterranean populations can be grouped into four main clusters: North Africa, Arab, Central-East, and West Mediterranean. The North African cluster exhibits high frequencies of North African specific haplotypes within haplogroup E3b. The Arab cluster exhibits high frequencies of J*(xJ2), which is rarer elsewhere.

According to the authors, there has been very little gene flow from North Africa into Europe. Moreover, Near Eastern populations should not be considered a unity, but are differentiated depending on the extent of Arab admixture exemplified by J*(xJ2) chromosomes. Modern Near Easterners are thus not representative of the early Neolithic people who migrated into Europe. J*(xJ2) chromosomes associated with Arabs are also present in North Africans, but North Africans have maintained their own Y-chromosomal peculiarities, typified by haplogroup E3b haplotypes.

It is unfortunate that a mainland Greek sample was not included, but to make up for it, there is a Cypriot sample, in addition to three Sicilian samples. These populations which are largely of Greek origin are very similar to Greeks in general, and belong to the Central-East cluster. Their inclusion also allow us to test my previously expressed hypothesis that haplogroup R1a1 was rare in ancient Greek populations. Indeed, this haplogroup is found at a frequency of 1.8-3.1% in Sicilians, Cypriots and Southern Italians, thus essentially confirming my idea. On the other hand, haplogroup I*(xI1b2) is found at frequencies from 3.4-15.7%, and is thus (as I have said before), much more likely to have been present in the ancient Greek population.

The study also examines briefly the origins of the Jews. Sephardic Jews are shown to resemble Mediterraneans more, while Ashkenazi resemble Arabs more.

The table of frequencies also allows us to ascertain the prevalence of Negroid admixture in Sicily, a popular subject in certain circles, and one which is shown to be without any basis in fact. In 212 Sicilians in total, no haplogroup A, E3a, or E*(xE3a,E3b) chromosomes were detected. Two haplogroup A chromosomes were detected in Cyprus, one in Sardinia, and two E3a, E*(xE3b, E3a) chromosomes in Malta. This is about the extent of male Sub-Saharan African introgression in the Mediterranean: 5 out of 656.

From the conclusions:

The significant genetic structuring of populations facing the Mediterranean basin into three groupings, Near Eastern Arab, Mediterranean and North African, is related to the demographic processes that have occurred since first populating the area. The distribution of Neolithic technologies was probably paralleled by demographic expansion in the Mediterranean basin, and subsequent westward migration by Phoenicians and Greeks contributed to the distribution of Y chromosome types of most likely Near East origin. The Arab conquest in particular appears to have had a dramatic influence on the East and South Mediterranean coasts, with differential sex-related gene flow playing a major role in the distribution of genetic variation. The presence of Arab Y chromosome lineages in the Middle East suggests that most have experienced substantial gene flow from the Arabian peninsula. This result raises the issue of the correctness of identifying all Near Eastern populations as reliable representations of the original Neolithic groups that expanded from the Middle East towards the European peninsula.

Annals of Human Genetics (online early)

Population Structure in the Mediterranean Basin: A Y Chromosome Perspective

C. Capelli et al.

Abstract

The Mediterranean region has been characterised by a number of pre-historical and historical demographic events whose legacy on the current genetic landscape is still a matter of debate. In order to investigate the degree of population structure across the Mediterranean, we have investigated Y chromosome variation in a large dataset of Mediterranean populations, 11 of which are first described here. Our analyses identify four main clusters in the Mediterranean that can be labelled as North Africa, Arab, Central-East and West Mediterranean. In particular, Near Eastern samples tend to separate according to the presence of Arab Y chromosome lineages, suggesting that the Arab expansion played a major role in shaping the current genetic structuring within the Fertile Crescent.

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